Morphofunctional state of the thyroid gland (TG) of young rats after exposure of exogenous melatonin in the spring and autumn periods was investigated. Exogenous melatonin (Unipharm Inc., USA) was administered to experimental group of animals daily at a dose 5 mg/kg. The duration of the experiment was 28 days. It was shown an increase of the cross-section area of follicles by 31 % and colloid by 30 % (in spring), reduction of the area of follicle epithelium by 12 % (in autumn), an increase in the follicle internal diameter and a decline in the thyroid epithelium height by 12 % (in autumn) in TG of experimental groups rats. Also it was shown a decline of follicle-colloid index and growth of colloid accumulation index, reduction amount of interfollicular islets. The introduction of melatonin in the spring period brought down the amount of connecting tissue elements. Melatonin introduction in the autumn period reduced the thickness of connective tissue trabecules in TG. The introduction of melatonin in the spring period resulted in more substantial changes in the structure of TG, as compared to an autumn experiment. Thus, melatonin administered both in the spring and in the autumn periods reduces the functional activity and physiological regeneration of TG.